1. Field of the Invention
The present invention relates to a cathode ray tube used for a computer monitor, a television receiver or the like, and the present invention relates particularly to a surface treatment technique thereof.
2. Description of the Related Art
Generally, with respect to a cathode ray tube, due to a thickness difference between the center and the periphery of the face portion for securing a vacuum pressure-resistant strength, or a phenomenon in which a width and a size or a film thickness condition of a phosphor stripe or a phosphor dot deteriorates as it approaches the periphery of the face portion, or also due to a color displacement of phosphors caused by the broadening of a deflection angle in the periphery of the face portion and so on, the face portion of the panel tends to have lower emission luminance in the periphery than in the center.
As measures against this, a technique is known in which a colored layer is formed on an outer surface of the face portion with a high density in the central portion and a low density in the peripheral portion of the face portion so as to allow the emission luminance of the cathode ray tube (luminance in the case of displaying the entire screen of the cathode ray tube with a certain signal) to be equal over the entire area of the face portion.
However, when the emission luminance is allowed to be equal over the entire area of the face portion by changing the density of the colored layer in this way, a darkness difference of the colored layer is increased between the center and the periphery of the face portion, so that a phenomenon is likely to occur in which the center is strangely darkened against the periphery, causing a contrast difference in images.
In particular, with respect to a cathode ray tube having a face portion whose outer surface is flat, which has been growing in demand in recent years, the periphery and the center of the face portion are located on the same plane, so that such a darkness difference of colors between the center and the periphery will be more noticeable.